This invention pertains to a method and apparatus for transferring and aligning integrally leaded semiconductor chips with conductive lead frame structures for permanently bonding thereto. More particularly, it involves the use of a magnetic force to raise the chip from a temporary carrier into engagement with an overlying lead frame and to concurrently automatically orient the chip while it is in transit, so that integral leads on the chip are precisely aligned with corresponding fingers of the lead frame.
One of the most time consuming and expensive steps in producing packaged semiconductor integrated circuit devices has been in the attachment of larger conductive leads to the integrated circuit chip to provide electrical connection to external circuitry. One technique is to bond filamentary wires which interconnect particular regions on the chip to corresponding leads of a conductive pattern on a rigid substrate, thereby making the electrical connection therebetween.
One can avoid the time consuming and expensive wire bonding operation by providing integral metal leads extending outwardly from one face of the chip. In a flip chip these leads extend perpendicular from the chip face, as contact bumps. In a beam leaded chip, the leads extend parallel to the chip face and are cantilevered over the edge of the chip. Such integral leads are extensions of a conductor pattern on the chip face, and serve as electrical interconnection points for larger conductive leads.
The larger conductive leads can be initially part of a lead frame having a plurality of spaced sets of inwardly convergent cantilevered fingers. The fingers of each set have free inner end portions which correspond to the integral lead pattern on the chip. In the usual lead frame constructed entirely of metal, the cantilevered fingers are unsupported at their free ends. Chips having contact bumps, popularly referred to as flip chips, can be attached to the lead frame by soldering the contact bumps directly to the corresponding finger free ends of the lead frame. This simultaneously produces a low resistance electrical connection between the bumps and respective fingers of the lead frame. While this alleviates the necessity for wire bonding, the chip still must be very precisely aligned so that the contact bumps are registered with their corresponding lead frame fingers.
One prior method of attaining this alignment involves supporting the back side of a flip chip on a small, needle-like vacuum chuck, and bringing the chip into engagement with overlying fingers of a lead frame extending parallel to the major surfaces of the chip. An operator manually raises and at least rotates the chuck while looking through a microscope to obtain good aligned contact bump-finger engagement. In many instances, the operator must also laterally move the chuck to get the proper register. This was time consuming, and therefore in high volume production proved to be expensive. In addition to the expense of such operations, it was inherently subject to human error due to the extremely small size of the chips. Moreover, the yields and reliability of the resultant devices depended upon the skill and precision of the individual operator, who before this invention had to manually align the chip and lead frame fingers.
In high volume production operations, the unsupported free ends of the converging lead frame fingers are frequently bent somewhat in handling so that they are not always precisely coplanar with one another. Furthermore, the height from contact bump to contact bump on an individual chip can vary. Therefore, substantial overtravel of the chuck is often needed to insure that every contact bump on a chip is intimately engaged with its corresponding finger. This chuck overtravel bends at least some and generally most of the fingers above their original plane. If the chip is bonded to the fingers while they are overstressed, a poor mechanical and electrical bond between the flip chip and the lead frame may result.
Through the use of our invention, integrally leaded semiconductor device chips can be automatically precisely aligned with lead frame structures without the necessity of manual alignment, thereby resulting in greater production efficiency and more reliable, stronger bonds.